Collects the panel background <rect> elements of a Plotly .bglayer,
deduplicated by position and sorted row-major (top-left first).
Shared by the normalizer (to wrap rects in <g id="axes_…"> groups) and
the extractor (to verify that the kept-panel count matches the rendered
background count before emitting subplot selectors).
Collects the panel background
<rect>elements of a Plotly.bglayer, deduplicated by position and sorted row-major (top-left first).Shared by the normalizer (to wrap rects in
<g id="axes_…">groups) and the extractor (to verify that the kept-panel count matches the rendered background count before emitting subplot selectors).